A first trip to the world of particle physics

Transcription

1 A first trip to the world of particle physics

2 Itinerary Massimo Passera Padova - 13/03/2013 1

3 Massimo Passera Padova - 13/03/2013 2

4 The 4 fundamental interactions! Electromagnetic! Weak! Strong! Gravitational Electromagnetic interactions: Hold atoms and molecules together Explain all em & optics Infinite range Mediated by the photon (QED) Massimo Passera Padova - 13/03/2013 3

5 EM: Maxwell equations µ F µν = 4π/c J ν ε µνρσ µ F ρσ = 0 Unifying electricity and magnetism, Maxwell explained the nature of light: optics became a branch of electromagnetism. These equations mark the beginning of the departure of physics from Mechanism. Massimo Passera Padova - 13/03/2013 4

6 Weak interactions decays Weak interactions: Muon decay Are at the origin of natural radioactivity Are weak, but only at low energy Mediated by W and Z (masses ~80 and ~90 times the proton mass) Distinguish between left- and right-handed particles (violate parity) Massimo Passera Padova - 13/03/2013 5

7 The 4 fundamental interactions! Electromagnetic! Weak! Strong! Gravitational Electroweak The strong interactions: hold atomic nuclei together confine quarks in protons and neutrons are mediated by gluons (QCD) are weak at short distances Massimo Passera Padova - 13/03/2013 6

8 The 4 fundamental interactions! Electromagnetic! Weak! Strong! Gravitational General relativity Einstein 1916 Are much, much weaker than the other interactions! We do not have a consistent quantum theory of gravitation Massimo Passera Padova - 13/03/2013 7

9 The 4 fundamental interactions! EM! Weak! Strong! Gravitational Electroweak GUTs? Strings?? All interactions are determinated by a symmetry principle (gauge symmetry) Is it possible to unify all forces using larger symmetries? Massimo Passera Padova - 13/03/2013 8

10 Massimo Passera Padova - 13/03/2013 9

11 Quantum Field Theory Massimo Passera Padova - 13/03/

12 Quantum Mechanics Describes the world of extremely small objects characterized by: Intrinsic uncertainty: there is a limit to the precision that can be reached in the simultaneous measurement of some observables. For example, if you measure an object, and determine the component p x = mv x of its momentum with an uncertainty p x, you won t be able, at the same time, to know its position x more accurately than x = (h/2π) / p x. This is Heisenberg s (famous) uncertainty principle. The constant h = J s is called the Planck constant. Common interpretation of waves and particles: the former can behave like the latter, and vice versa. Massimo Passera Padova - 13/03/

13 Theory of special relativity Describes the motion of an object modifying the predictions of Newtonian mechanics when its speed gets close to the speed of light: c ~ km/sec The speed of light in vacuum c is finite and does not depend on the motion of its source. It s the maximum speed. The effective mass increases with speed. Energy and mass are the same thing. Mass is just a form of energy Massimo Passera Padova - 13/03/

14 Symmetries In physics we speak of symmetry when a system is invariant under a certain transformation (e.g. of the coordinates). Symmetries can be discrete, or continuous Each continuous symmetry is associated with a conserved quantity. The space-time symmetries determine the constants of the motion (for example: translational invariance à conservation of momentum). Massimo Passera Padova - 13/03/

15 Symmetries determine the interactions C.N. Yang Today we believe that symmetries determine the fundamental laws of physics The modern theory of interactions of elementary particles is a quantum theory of gauge fields whose invariance is with respect to local generalized rotations in an internal space General relativity is based on a similar idea Massimo Passera Padova - 13/03/

16 All clear? Massimo Passera Padova - 13/03/

17 Is there anything clear? Massimo Passera Padova - 13/03/

18 Massimo Passera Padova - 13/03/

19 The Standard Model EM Weak Strong BOSONS spin 1 Higgs Boson FERMIONS spin 1/2 (there is no evidence, up to now, of any lepton or quark structure) Massimo Passera Padova - 13/03/

20 Three generations: the particle zoo reduced to fundamental ingredients At first sight, only the first family shows up in everyday life The basic unit of energy is the electron Volt (ev): it is the variation of potential energy of a single electron that crosses a difference of potential of 1 Volt. in cosmic rays To observe the heaviest building blocks we need accelerators Massimo Passera Padova - 13/03/

21 New particles? 2 complementary ways: Direct production relativistic way but also Indirect research quantum way Heavy particles induce virtual effects ( quantum corrections) which are very tiny, but very useful too: top Higgs Example: the anomalous magnetic moment of the muon, measured with the fantastic precision of 0.5 parts per million (!), is sensitive to possible effects of physics beyond the SM Feynman diagram Sometimes indirect signals have anticipated direct discoveries. Eg: the top quark Massimo Passera Padova - 13/03/

22 Precision tests and the top quark Nov 1994: fits to precision measurements predict: M top = 178± GeV March 95: Fermilab discovers the top quark. Today it is: M top = 173.1±1.3 GeV Great success of SM and experimental program! Can we repeat this analysis with the Higgs boson? Yes, but the sensitivity to its mass is much weaker. More later Massimo Passera Padova - 13/03/

23 The lesson of LEP The Large Electron Positron Collider (LEP) ( ) at CERN did not discover new particles but confirmed the SM gauge interactions with a fantastic precision: 0.1%! Massimo Passera Padova - 13/03/

24 The LHC at CERN (Geneva) Massimo Passera Padova - 13/03/

25 CMS ATLAS Massimo Passera Padova - 13/03/

26 ATLAS: Animation of a real proton collision occurred in The two photons produced in the collision are indicated by their energy cluster (in green). ATLAS Experiment 2012 CERN. Massimo Passera Padova - 13/03/

27 Broken symmetries The SM unifies weak and electromagnetic interactions. If they are unified, why do they appear so different to us? Their range is very different. We believe that In nature it is not uncommon to have non symmetric states of minimum energy: for example, a ferromagnet. The (broken) symmetry manifests itself in the equivalence of the breaking options. Massimo Passera Padova - 13/03/

28 Spontaneous symmetry breaking Infinite # of minima. All equivalent, but distinct. We believe that this process of spontaneous symmetry breaking is at the origin of the mass of the particles à there are good chances that the LHC will help us understand it better. Massimo Passera Padova - 13/03/

29 Massimo Passera Padova - 13/03/

30 The mystery of the Higgs Peter Higgs Massimo Passera Padova - 13/03/

31 The Higgs boson There must exist something that breaks the symmetry of the vacuum and, interacting with the particles, gives them their masses. It could be an elementary field: the HIGGS boson (or the manifestation of something more elaborate) Today, the electroweak symmetry breaking mechanism is the central problem of particle physics Massimo Passera Padova - 13/03/

32 What did we know about the Higgs a few years ago? Direct searches at LEP: its mass is M H > GeV Comparing the experimental measurements with the SM predictions that depend on M H we obtained indirect information on M H : clear preference for a light Higgs (below ~150 GeV) Massimo Passera Padova - 13/03/

33 The Higgs has been found! (or not?) July 2012: ATLAS and CMS announce the discovery of a new particle with mass of about 125 GeV (*) and properties consistent with those of the Higgs boson predicted by the SM: Is it really the SM Higgs? It s too early to say, but the LHC data will tell us! (*) the mass of the proton is about 1GeV Massimo Passera Padova - 13/03/

34 CMS: The event shows characteristics expected from the decay of the SM Higgs boson to a pair of photons (dashed yellow lines and green towers). The event could also be due to known SM background processes. CMS Experiment 2012 CERN. Massimo Passera Padova - 13/03/

35 ATLAS: Candidate event for the decay of a SM Higgs boson into 4 muons (in red). The origin of the 4 muons is a common primary vertex. ATLAS Experiment 2012 CERN. Massimo Passera Padova - 13/03/

36 Massimo Passera Padova - 13/03/

37 Why don t we believe in the SM?? The discovery of the Higgs is part of a more ambitious project to understand the origin of the symmetry breaking that generates the masses, and the nature of the electroweak scale. The Standard Model passed so many tests, however: Too many parameters. Why 3 families? Incomplete: what about gravity? Why is it so weak? Doesn t explain the smallness of neutrino masses Doesn t explain the observed dark matter (and energy), nor baryogenesis. The SM must have a completion that we still don t know. Supersymmetry? Extra dimensions..?? Massimo Passera Padova - 13/03/

38 Massimo Passera Padova - 13/03/